1. Field of the Invention
The present invention relates to a cooling method and more particularly to a method for the cooling of a heated flow containing solid and/or semisolid product particles of foodstuff character.
2. Description of the Related Art
In packing technology, aseptic packaging is frequently used for the packing of particularly bacteria-sensitive and storage-sensitive products, e.g. foodstuffs, which can be rapidly destroyed or seriously impaired if they are subjected to uncontrolled microbiological influences. Aseptic packaging in principle is based on the concept that the product which is to be packed should first be sterilized and subsequently be packed in separately sterilized packing containers, or in packing containers manufactured from sterilized packing material, under sterile or aseptic conditions so as to prevent a reinfection of the sterilized product. The aseptic packing technique has the number of advantages over other conventional, non-aseptic packing technique from a point of view of keeping property of the product as well as of distribution. This means, inter alia, that the aseptically packaged product, whilst preserving virtually unchanged freshness qualities can be stored during appreciably longer storage periods than is the case for a corresponding, non-aseptically packed product. Moreover, such storage of the aseptic product during the whole of its handling, from the actual packaging to the final consumer, can take place without any need for cold storage or an unbroken cooling chain, which appreciably facilitates the distribution of the product.
Since bacteriia and other microorganisms generally are very temperature-sensitive and rarely survive at temperatures above approx. 100.degree.-110.degree. C., it is customary for product sterilization on aseptic packages to be carried out by means of a controlled heat treatment. This means that the product, according to a predetermined time/temperature scheme, is heated to, and is maintained at, a temperature which is high enough for the desired killing or sterilizing effect, whereafter the product is cooled and is then packed. With the help of modern aseptic packing machines of the type which form, fill and close finished packages it is possible at present to carry out the whole aseptic packing procedure, including product sterilization, continuously on a large scale.
A heat treatment of the type which has been described above is both simple and effective from a point of view of sterilization, and in general functions well with foodstuffs of the type of milk, juice or other liquid or pumpable, homogeneous products. Treatment may, however become more problematic and difficult to carry out with more heterogeneously composed foodstuffs which contain both solid and liquid components. Examples of products of the last-mentioned heterogeneous type are, for example, meat broths, vegetable soups and similar mixtures of sauces/juices and meat/fruit pulp particles. Foodstuffs, whether they are in solid or in liquid form, are heat-sensitive and may easily be damaged by a heat treatment if the same is too extensive and/or is performed at too high a temperature. In order to avoid such a damaging heat effect it is important, therefore, for the sterilizing heat treatment to be carried out at a predetermined temperature which is, right for the particular foodstuff. On the one hand, the temperature should be sufficiently high so as to ensure a sterilizing effect, but on the other hand the temperature should not be high enough for the foodstuff to suffer damage or even risk suffering damage by the treatment. The extent of the heat treatment, moreover, should be adapted to keep the total heat load on the foodstuff within such limits that the smallest possible negative effect on the qualities of the product is assured. This means in practice that the actual heating process to the desired sterilization temperature preferably should be as rapid as possible, and that the cooling process after the sterilization ought to be carried out rapidly also so that the total heat load on the foodstuff can be kept to a minimum without the sterilization effect being neglected.
With the help of so-called ohmic heating, which is based on converting electric energy into thermal energy directly in the foodstuff by allowing an electrically conducting flow of the product to pass between oppositely charged electric voltage poles, it is now possible to achieve a very rapid heating and accordingly a limited heat load on the product inasfar as the heating part of the sterilizing process is concerned. However, it has proved to be more difficult to cool the heated product flow equally quickly, or at least sufficiently quickly, in order to effect minimization of the total heat load on the product during cooling too. The problem in particular involves highly viscous, heterogeneous product types containing both liquid and solid or semisolid components of the afore-mentioned type, and to a large extent is connected with, or is due to, the fact that as a rule it is much easier to cool the liquid element in the product than the solid or semisolid components. The solid or semisolid components require longer cooling periods, and thereby limit the possibility of carrying out the cooling process within the desired minimum amount of time. The drawn-out cooling process and the increased heat load connected therewith often has led to the solid and/or semisolid components of the finished product having been subjected to an impairment of quality which expresses itself, among other things, in a partially lost nutritional content, impaired chewing resistance, and also generally negatively affected organoleptic properties. At the same time, the relatively long cooling transport distances which the product is by necessity forced to travel by pumps, contributes further to an impairment of the qualities of the product due to the pumps mechanical effect on the product.